A diplomatic cable leaked to an online magazine said five people had died in the attack on December 23, with the symptoms fitting those of poisoning by a nerve gas known as BZ or by its code-name Agent 15.

Videos posted online showed the victims struggling for breath and choking on their own vomit – the cause of death in all the lethal cases. A number of other people were affected to varying degrees, said the magazine, Foreign Policy, which was briefed on the document conducted its own interviews with the doctors who reported them.

"It was a chemical weapon, we are sure of that, because tear gas can't cause the death of five people," said one, Dr Nashwan Abu Abdo. He said some of the patients suffered mental confusion including hallucinations and behaviour changes, and some had recurrent symptoms.

Although tear gas can cause choking, it cannot cause this range of effects.

"They all had miosis – pinpoint pupils," the doctor added. "They also had generalised muscle pain. There were also bad symptoms as far as their central nervous system. There were generalised seizures and some patients had partial seizures."

The anonymous diplomat who leaked the document said: "We can't definitely say 100 per cent, but Syrian contacts made a compelling case that Agent 15 was used in Homs on December 23."

The study is unlikely to be held as conclusive by the White House, which tried to play down the report.

"The reporting we have seen from media sources regarding alleged chemical weapons incidents in Syria has not been consistent with what we believe to be true about the Syrian chemical weapons programme," the National Security Council spokesman, Tommy Vietor, said.

President Barack Obama and his senior officials have said that the use of chemical weapons would be a "red line", triggering international intervention. He has already shifted that red line back – it was originally said to be placed at a point where Syria started moving or preparing chemical weapons, but a Pentagon allegation that sarin gas was being prepared was taken no further after the activity suddenly stopped.

The White House's doubt may relate to the fact that Syria's known chemical weapons stocks include substances like sarin and VX which are far more deadly than Agent 15. The State Department made clear it believes those stocks are still secure.

Sergei Lavrov, the Russian foreign minister, who is close to the Syrian regime, said last month its main chemical arsenal had been withdrawn to two secure locations.

Foreign Policy speculated that if the use of Agent 15 were real, it might have been an attempt by Mr Assad to test how far he could push Mr Obama, whose lack of willingness to intervene in Syria has if anything grown as the war has intensified.

Georges Sabra, deputy head of the opposition Syrian National Coalition, complained that despite promises of backing and recognition from western powers, little real support was arriving.

"The friends of the Syrian regime are providing it with the tools to kill, and the friends of the Syrian people are giving the regime the time it needs," he told a Saudi newspaper.

Meanwhile, the killing on the ground continued, including in Homs, where the long government siege of rebel-held districts continues. In Idlib city, which remains in the hands of the regime despite being surrounded by large stretches of rebel-held territory, three car-bombs killed 22 civilians, according to state media.

Opposition activists said 24 people died in the explosions, mostly military.